Not able to pull off simple combination of volume scatter and volume absorbtion: cycles
Wrong Add node

The Math (Add) node you are using works with grayscale values (on the left in the image below).

You would like instead to Add a shading to another shading: you must use the Add Shader node you can find in the Shader nodes group (on the right in the following image).



The color of the input/output socket is always a guide for the correct connection.